The present invention relates generally to a method of bleaching and delignifying lignocellulosic pulps. More particularly, the invention relates to a method of bleaching and delignifying lignocellulosic pulps with ozone.
In recent years the pulp and paper industry has devoted substantial efforts to the development of chlorine-free or reduced chlorine bleaching processes. A milestone in this long range effort has been the development and implementation of oxygen bleaching systems generally, and particularly the low consistency oxygen bleaching process described in Roymoulik et al., U.S. Pat. No. 3,832,276.
Oxygen bleaching, however, is only a partial solution to the chlorine-free concept, since oxygen bleaching by itself cannot produce pulps of sufficiently high brightness and quality. One route which has been investigated by a number of researchers is to employ ozone as a bleaching agent, either alone or following an oxygen bleaching stage, since ozone has the capability of bleaching pulps to high brightness and is free of chlorine. Recent publications relating to ozone bleaching of wood pulps include Rothenberg et al. (Tappi 58 [8] 1975: 182); N. Soteland and K. Kringstad (Norsk Skogindustri [2] 1968:1); R. B. Secrist and R. P. Singh (Tappi 54 [4] 1971:581); Z. Osawa and C. Schuerch (Tappi 46 [2] 1963:79). U.S. Pats. relating to the ozone bleaching of fibrous materials, include: Nos. 396,325, 1,760,042, 1,957,937, 2,466,633, 3,049,394, 3,149,906, 3,318,657, 3,352,642, 3,451,888, 3,663,357, 3,806,404, and 3,829,357, and Canadian Pat. Nos. 902,861, 966,604 and 970,111.
The above-cited literature and patent references generally describe gas phase ozone bleaching of fibrous materials having consistencies in the range of 15% to 75%. The references disclose that the optimum conditions for ozone bleaching fall within the range of 30% to 60% consistency. When operating at such consistencies, a number of advantages are realized. Among these advantages are reaction times of relatively short duration, namely, from 1 minute to 60 minutes, and acceptable ozone utilization. However, there are also a number of disadvantages associated with high consistency processes for bleaching pulp with ozone. These include: (1) the necessity of employing expensive process equipment, e.g., high consistency presses, pulp fluffers and high consistency stock pumps; (2) the danger of pulp fires at these relatively dry conditions; and (3) difficulty in de-gassing pulp after treatment with ozone which can result in severe washer operation problems.
In an attempt to avoid the difficulties associated with ozone bleaching at high consistency, a number of investigators have experimented with ozone bleaching at low pulp consistency. The ability to successfully employ pulp at low consistency in ozone bleaching would avoid the difficulties described above.
Accordingly, Osawa and Schuerch (Tappi 46 [2] 1963:79) compared ozone bleaching of kraft pulp at 1% pulp consistency and 50% pulp consistency, respectively, at neutral pH. They concluded that the rate of ozonation is highest when it is carried out in the gas phase on fibers well above the fiber saturation point, i.e., 19%-20% consistency, and is much slower when the fibers are suspended in water.
Soteland and Kringstad (Norsk Skogindustri [2] 1968:1) confirmed the findings of Osawa and Schuerch when treating mechanical pulps at neutral pH with 2.25% ozone (w/w) in oxygen. They observed that, "Preliminary investigations showed that ozone passes through a suspension of mechanical pulp in water (concentration for instance between 0.1 and 0.2%) without being consumed to any significant degree." They found that the addition of 10% (v/v) of a suitable organic solvent, such as methyl or ethylacetate, acetic acid or acetone to the suspension was necessary to catalyze the reaction of ozone with the pulp. Similarly, Osawa and Schuerch also observed and noted that the presence of nitromethane or methylacetate serve to catalyze the reaction of ozone with pulp.